Reduction of the angle-modulation of amplitude-modulated oscillations



Dec. 29. 1953 E. MCP. LEYTON REDUCTION oF THE ANGLE-MODULATION or"AMPLITUDE-MODULATED OSCILLATIONS Filed May 2, 1950 OSC. /"2

/nven/r ERIC MCPHAIL LEYTON @Y #Horny :z//r'aed Dec. 29, `i953 UNITEDSTATS FTENT- ZtlliS OFFl E REDUCTION OF THE ANGLE-MODULA- TION FAIVIPLITUDE-MODULATED GSCILLATIONS England, a company of Great BritainApplication May 2, 195i), Serial No. 159,414

Claims priority, application Great Britain May 3, 1949 2 Claims.

This invention relates to circuits for reducing the angle-modulation oi'amplitude-modulated oscillations. It is sometimes required to removephase and/or frequency modulation from amplitude-modulated carrieroscillations, so as to provide oscillations which are substantially onlyaznplitude-modulated. Such a reuuirememl may arise in the case wheresingle sideband amplitude-modulated oscillations are required to bedealt with, the removal of one of the sidebands resulting in theresultant oscillations being anglemodulated.

Tlie object of the invention is to provide an improved circuit for thispurpose.

According to the invention there is provided a circuit for removingangle-modulation, ccmprising a source of amplitude-modulatedoscillations having undesired angle-modulation, means for generatingoscillations o constant amplitude and of a constant frequency which isdifferent from the frequency oi said modulated oscillations, means forcombining said amplitude-modulates` oscillations with said constantfrequency oscillations to provide further oscillations of frequencieswhich are the sum and difference frequencies of said modulatedoscillations and said constant frequency oscillations, means forselecting, from said further oscillations, one of said kinds vofoscillations, means for combining said selected oscillations withsaidamplitude-modulated oscillations to provide ,resultant oscillationsat fre.- quencies which are the sum and difference fre.- quencies ofsaid selected oscillations and said modulated oscillations, and meansfor Sele-stime, from said resultant oscillations, oscillations at saidconstant frequency ...amplitude-modulated and substantially free fromangle-modulation,

In order that the said invention may be clearly understood and readilycarried into effect, the same will now be more fully described withreference to the accompanying drawing, Figure l oi.

which is a block diagram of a circuit according to oneiorm of theinvention, and Figure 2 illustrates a circuit arrangement suitable foruse according to the invention.

In the embodiment of the invention shown in the drawing, the circuit isintended to remove angle-modulation from amplitude-modulated carrieroscillations o1" reouency Fc. The amplilaude-modulated carrieroscillations are icd to the input circuit of a modulator i, Figure Landother oscillations of constant amplitude and of predetermined frequencyF1, preferably 01"' lower frequency than the frequency Fe, and generatedby a local oscillator '2 are also fed to modulator i. The oscillationsare combined in modulator l so as to provide in the output of modulatorl further oscillations of frequencies Fe-i-FL and Fer-FL. This output isfed to a lter 3, which passes only signals of frequency Fri-F1. andsidebands thereoi. The output from iilter 3 is fed, in some cases via alimiter das will be described hereinafter, to a second modulator 5 towhich are also fed the carrier oscillations of frequency Fe. Theoscillations fed to modulator 5 are combined therein so as to provide inthe output of modulator 5 resultant oscillations of frequency Fc+Fz.-Fc--Fr., and Fc-l-FL-i-Fc. This output is applied to a lter S which passesonly signals of frequency F1. and

sidebands thereof.

Thus the output of lter 6 comprises oscillations of the same frequencyas the oscillations generated in oscillator 2, and any .change in thephase or frequency of the amplitude-modulated carrier oscillations willnot produce any corresponding .change in the frequency of theoscillations in the output of filter t, with the result that theangle-modulation .of the .carrier oscillations is eiectively removed.

Amplitude-modulation ol the carrier oscillations will, however, resultin amplitude-modulation of the output from filter e in the same sense.4The law connecting the way in which the amplitude of said output changesWith the amplitudemodulation of the, carrier oscillations, will ofcourse depend upon the type of circuit employed for the modulators l and5. The amplitude modulation is of course to be distinguished from the.'

actual amplitude of the carrier oscillations, which may be modied by thepresence of phase or irequency modulation. It 1s generally required that'the amplitude-modulation of the carrier ,oscillations should bereproduced Without distortion, that is to say, there should oe a linearlaw between the input and output amplitudes ci' the circuit. This can oeachieved by employing modulators having a linear laW relating input and.output. If, however, the modulators have a different law, for example asquare law, then the amplitude-modulation of the output oi the circuitis liable to distortion due to the mixing oi' the carrier oscillationsand said further oscillations in modulator 5. Said distortion can bereduced by feeding the output from filter 3 to modulator 5 via a limiterl which in effect modifies the law o the modulators in such sense thatthe circuit has a linear law. Instead of employing limiter themodulators I and 5 may be arranged to operate as a limiter by arrangingIthat the input to modulator l from oscillator 2 iS small compared withthe carrier oscillations and that the input to modulator 5 from lter 3is large compared with the carrier oscillations applied to modulator 5.

If desired filter 3 may be arranged to provide an output comprisingoscillations of a frequency which is the difference between thefrequencies of the amplitude-modulated carrier oscillations and theoscillations generated by oscillator 2 and in this case lter is arrangedso that its outputvv 3 comprises oscillations of a frequency which isthe sum of the frequencies of the amplitudemodulated oscillations andthe oscillations fed to modulator 5 from filter 3.

A circuit according to the invention, since it removes phase andfrequency modulation from oscillations, will give a double sidebandoutput when fed with a single sideband or vestigial sideband input. Itcan therefore be advantageously employed in a television receiver forthe reception or' vestigial sideband signals and will prevent thedistortion which would otherwise occur due to the effect known asasymmetric sideband overshoot. Said distortion occurs in single sidebandtransmission and also in the transmission of double sideband signals bycable, the signals fed from the cableinevitably being of vestigialsideband nature due to the frequency response of the cable.

It is to be understood that the term anglemodulated employed hereinrefers not only to oscillations which have been purposely modulated inphase or frequency but also to oscillations in which angle-modulationoccurs as a form of distortion.

Figure 2 of the accompanying drawing illustrates a detailed circuitarrangement in accordance with the block diagram oi' Figure 1. In Figure2, the reference numeral `I indicates a source of amplitude-modulatedoscillations which may be vestigial sideband oscillations havingundesired phase or frequency modulation. rThe oscillations are fed tothe control grid 8 of a modulator tube 55 through a coupling condenserIii, the control grid 8 being connected to ground through a leakresistance il. The oscillations iedto the controi grid u are theoscillations 11e referred to 'in connection with Figure 1. oscillationsof a 'irequency Fi. are fed to another grid I2 of the tube 9 from anoscillator comprising a tube I3 which, together with the associatedelements shown, constitute a conventional iloipitts oscillator Whichneed not be described in further detail. The oscillations set up at theanode of the tube I3 are fed to the control grid l2 trirough a couplingcondenser hi, the control `grid being connected to earth by a leakresistance lo. ihe anode oi the tube i3 is supplied Witii ano-uepotential trom 'une lead iii which is connected to the positive teiminalof a source oi' H. 'i'. potential, tiiiougn a radio frequency cnoke la.'ine two .screening electroues of the tube are also connected to thelead Iii and are decoupled to eartn through a oecoupling condenser i8.The anode oi' the tube 9 is connected to a iilter consisting or a pairof tuned circuits I9 and 2u corresponding to the filter 3 of Figure 1which serves to provide an output of frequency Fc-l-Fi.. The tunedcircuit i9 is connected to the lead i6, whereby anode potenv'tial issupplied to the tube S, the anode being decoupieu to earth through adecoupling coiidenser El. The high potential end or the tuned circuit 2dis connected to the control electrode 22 of a limiter tube 23 through acoupLng ctndenser 24. 'ihe control grid :i2 is connected to earththrough a leak resistance 25. The tube 2S constitutes the limiter l ofFigure 1 and is shown as a screen grid tube with the screen electrode 25connected to the lead I6 and decoupled by a decoupling condenser 2. Theanode of the tube 23 is connected by a coupling circuit consisting of apair of tuned circuits 28 and 29, which are arranged to pass thefrequency Fc-l-FL, to one control grid 3i) of a modulator tube 3l whichconstitutes the modulator 5 shown in Figure 1; The

circuit Mis-connected to the lead (6 whereby .anode potential issupplied to the anode of the tube 23, the latter being decoupled by adecoupling condenser 32. The other control grid 33 f of the tube 3l isconnected to the source 'I by a lead 34 so that oscillations offrequency Fc are supplied to the grid 33 which is connected to groundthrough a leak resistance 35. The screening electrodes of the tube 3|are connected to the lead I5 and are decoupled by a decoupling condenser36. The anode of the tube 3| is connected to a circuit 3l tuned to thefrequency Fi. and constitutes the lter S of Figure 1, this tuned circuit3l being connected to the lead i6 whereby anode potential is supplied tothe anode of the tube 3l which is decoupled by a decoupling condenser38. A coil 39 is coupled to the coil of the tuned circuit 31 to providea double sideband radio frequency output as described in connection withFigure 1.

What I claim is:

l. A circuit for removing angle-modulation, comprising a source ofamplitude-modulated oscillations having undesired angle-modulation,means for generating oscillations of constant amplitude and of aconstant frequency dilferent from the frequency of said amplitudemodulated oscillations, means for combining said amplitude-modulatedoscillations with said constant frequency oscillations to providefurther oscillations of frequencies which are the sum and differencefrequencies of said modulated oscillations and saidconstant frequencyoscillations, means for select-- ing one of said kinds of oscillationsfrom said iur# ther oscillations, means for combining said selectedoscillations with said amplitude-modulated oscillations to provideresultant oscillations at frequencies which are the sum and diierencefrequencies of said selected oscillations and said modulatedoscillations, and means for selecting, froinisaid resultantosciliations, oscillations at said constant frequencyamplitude-modulated and substantially free from angle-modulation.

2. A circuit for removing angle-modulation, comprising a source ofsingle side-band amplitude-modulated oscillations, means for generatingoscillations of constant amplitude and of a constant frequency differentfrom the frequency of said modulated oscillations, means for combiningsaid amplitude-modulated oscillations with said constant frequencyoscillations to provide further oscillations of frequencies Lwhich arethe sum and diiierence frequencies of said modulated oscillations andsaid constant frequency oscillations, means for selecting one of saidkinds of oscillations from said further oscillations,

means for combining said selected osciliations ERIC MCPHAIL LEYTON.

References Cited in the le of this patent UNITED STATES PATENTS NameDate Lewis June 26, 1934 Fritz i..-- Oct. 20, 1942 Number

